1. Field of the Invention
In general, the present invention relates to the production of a synthetic natural zeolite, namely clinoptilolite, and in particular a sodium clinoptilolite.
2. Description of the Prior Art
Certain naturally occurring hydrated metal aluminum silicates are called zeolites. The synthetic zeolites of the invention have compositions similar to those of the natural zeolite, clinoptilolite. Clinoptilolite is useful in feeding animals, as a hydrocarbon conversion catalyst and for water purification.
Zeolites consist basically of a three-dimensional frame work of SiO.sub.4 and AlO.sub.4 tetrahedra. The tetrahedra are crosslinked by the sharing of oxygen atoms so that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to two or O/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, a sodium ion. This balance may be expressed by the formula Al/Na=1. The spaces between the tetrahedra are occupied by water molecules prior to dehydration.
The composition of all zeolites may be represented by the general formula as follows: EQU (M.sub.2.sup.+,M.sup.2+)O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
M.sup.+ is usually Na or K, and M.sup.2+ is Mg, Ca or Fe and more rarely another of the group I or group II elements. In general, a particular crystalline zeolite will have values for "x" and "y" that fall in a definite range. The value "x" for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. It is generally equal to or greater than 2. Minor variations in the relative number of these atoms do not significantly alter the crystal structure or physical properties of the zeolite. The value of "y" is not necessarily an invariant for all samples of zeolites. This is true because various exchangeable ions are of different size, and since there is no major change in the crystal lattice dimensions upon ion exchange, the space available in the pores of the zeolites to accommodate water molecules varies.
The formula for sodium clinoptilolite may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O.Al.sub.2 O.sub.3.(5.2-10.2).+-.0.1SiO.sub.2.yH.sub.2 O
wherein "y" may be any value up to 7.
The pores of zeolites normally contain water.
The above formulas represent the chemical analysis of clinoptilolite. When other materials as well as water are in the pores, chemical analysis will show a lower value of "y" and the presence of other absorbates. The presence in the crystal lattice of materials volatile at temperatures below about 600.degree. C. does not significantly alter the usefulness of the zeolites as an absorbent since the pores are usually freed of such volatile materials during activation.
Among the ways of identifying zeolites and distinguishing them from other zeolites and other crystalline substances, the X-ray powder diffraction pattern has been found to be a useful tool. In obtaining X-ray powder diffraction patterns, standard techniques are employed. The radiation is the doublet of copper, and a Geiger counter spectrometer with a strip chart pen recorder is used. The peak heights, I, and the positions as a function of 2.theta. where .theta. is the Bragg angle, are read from the spectrometer chart. From these, the relative intensities, 100I/I.sub.o, where I.sub.o is the intensity of the strongest line or peak, and "d" the inter planar spacing in .ANG. corresponding to the recorded lines are calculated. A clinoptilolite type zeolite has a characteristic X-ray powder diffraction pattern which may be employed to identify it. The X-ray powder diffraction data are shown in Table A. The values for the interplanar space "d" are expressed in angstrom units.
TABLE A ______________________________________ X-RAY DIFFRACTION PATTERN FOR SYNTHETIC ZEOLITE OF CLINOPTILOLITE TYPE Relative hkl d(.ANG.) Intensity ______________________________________ 020 8.92 100 002 7.97 3 101 6.78 2 031 5.61 2 112 5.15 7 130 4.65 14 103 4.35 2 132 3.964 55 004 3.964 55 042 3.897 57 141 3.74 7 211 3.55 6 051 3.48 3 220 2.419 16 202 2.324 4 222 3.168 14 222 3.119 15 231 3.07 8 044 2.974 80 035 2.793 15 125 2.793 15 161 2.728 33 ______________________________________
Occasionally, additional lines not belonging to the pattern for the zeolite appear in a pattern along with the X-ray lines characteristic of that zeolite. This is an indication that one or more additional crystalline materials are mixed with the zeolite in the sample being tested. Frequently, these additional materials can be identified as initial reactants in the synthesis of the zeolite, or as other crystalline substances. When the zeolite is heat treated at temperatures of between 100.degree. C. and 600.degree. C. in the presence of water vapor or other gases or vapors, the relative intensities of the lines in the X-ray pattern may be appreciably changed from those existing in the unactivated zeolite patterns. Small changes in line positions may also occur under these conditions. These changes in no way hinder the identification of these X-ray patterns as belonging to the zeolite.
The particular X-ray technique and/or apparatus employed, the humidity, the temperature, the orientation of the powder crystals and other variables, all of which are well known and understood to those skilled in the art of X-ray crystallography or diffraction can cause some variations in the intensities and positions of the lines. These changes, even in those few instances where they become large, pose no problem to the skilled X-ray crystallographer in establishing identities. Thus, the X-ray data given herein to identify the lattice for a zeolite, are not to exclude those materials, which, due to some variable mentioned or otherwise known to those skilled in the art, fail to show all of the lines, or show a few extra ones that are permissible in the cubic system of that zeolite, or show a slight shift in position of the lines, so as to give a slightly larger or smaller lattice parameter.
A simple test described in "American Mineralogist", Volume 28, Page 545, 1943, permits a quick check of the silicon to aluminum ratio of the zeolite. According to the description of the test, zeolite minerals with a three-dimensional network that contains aluminum and silicon atoms in an atomic ratio of Al/Si=2/3-0.67, or greater produce a gel when treated with hydrochloric acid. Zeolites having small aluminum to silicon ratios disintegrate in the presence of hydrochloric acid and precipitate silica. These tests were developed with natural zeolites and may vary slightly when applied to synthetic types.
Clinoptilolite may be useful as a second zeolite component in fluid catalytic cracking (FCC) catalysts. It would satisfy a need for high octane conversion of the light gasoline fractions, a need that ZSM-5 serves in some very crucial cases, but for which ZSM-5 has serious shortcomings. Clinoptilolite also holds promise in ammonia removal from waste water, reduction of ammonia toxicity in animal nutrition, and as a slow release ammonia component in soil nutrition fertilizers.
Clinoptilolites found in nature are impure and contain structural imperfections that make reliable property characteristics unobtainable. A uniform, high quality synthetic clinoptilolite would have considerable promise for a number of commercial applications.
It is a primary object of the present invention to provide a simple, economical process for making a synthetic crystalline zeolite of the clinoptilolite type.
Other objects and advantages of the invention will be more readily apparent from a reading of the description hereinafter.